1. Field of the Invention
The invention relates to a heat exchanger for two-stage heat exchange between a first fluid, one the one hand, and a second and third fluid having a different temperature, on the other hand. The heat exchanger can include a block for the separated and heat-exchanging conveying of the first and of the second and third fluid, a plurality of flow channels through which the first fluid can flow, a first chamber of a high-temperature part, whereby the chamber can accommodates the flow channels and through which the second fluid can flow, and a second chamber of a low-temperature part, said chamber which accommodates the flow channels and through which the third fluid can flow, and a housing in which the first and second chamber and the flow channels are arranged.
2. Description of the Background Art
Above all, exhaust gas recirculation (EGR), particularly cooled exhaust gas recirculation, is used in current automotive vehicles because of legal requirements to reduce particulate, pollutant, and particularly nitrous oxide (NOx) emissions. For this purpose, in an exhaust gas recirculation system, part of the exhaust gas is removed from the exhaust gas line at a suitable place, cooled, and returned to a motor on the fresh charge side. The EGR-related decline in partial oxygen pressure results in rather low peak combustion temperatures, which in turn result in lower formation rates for thermal NOx. The cooling of the returned exhaust gas intensifies the effect further. The cited principle has proven especially effective in the passenger vehicle sector.
An exhaust gas recirculation system of the applicant is described in greater detail, for example, in German Pat. No. DE 60 024 390 T2, which corresponds to U.S. Pat. No. 6,244,256, which is incorporated herein by reference, and which shows a single-stage exhaust gas cooler, which with the aid of a coolant circulation, coupled to the engine cooling water, can cool the exhaust gas, depending on the size of the exhaust gas cooler, to outlet temperatures up to the range of 110° C. A two-stage exhaust gas cooling is also described therein according to which behind a first high-temperature heat exchanger a second low-temperature heat exchanger is arranged, the former for recooling being coupled to a high-temperature cooling loop and the latter to a low-temperature cooling loop. The low-temperature cooling loop in this case can have coolant inlet temperatures in the range of 40-60° C. The temperature reductions achievable in the exhaust gas with two-stage heat exchangers are clearly above those for single-stage exhaust gas coolers. In the latter case, there is the problem that after a cold start, engine cooling water is heated relatively rapidly to temperatures of 90-110° C.
However, in the conventional art, an outlet temperature of a single-stage exhaust cooler can therefore be cooled at most to the inlet temperature of the engine cooling water, even with the assumption of ideal heat transfer. In order to achieve this, single-stage exhaust gas coolers usually have a relatively long space requirement. Two-stage exhaust gas coolers, as disclosed, for example, in German Unexamined Pat. Application No. DE 103 51 845 B4, prove to be relatively cost-intensive in realization because of the generally necessary high-temperature part and low-temperature part. Moreover, two-stage heat exchangers usually have a greater pressure loss than single-stage heat exchangers.
An improved structural design for a two-stage heat exchanger would be desirable. Designs of this type are disclosed by the applicant, for example, in German Unexamined Pat. Application No. DE 102 03 003 A1, which corresponds to U.S. Pat. No. 7,032,577, which is incorporated herein by reference, and in which a two-stage heat exchanger with a bypass channel is described in greater detail. The placement of a block with a high-temperature part and a low-temperature part for heat exchange in a common housing has the advantage that comparatively few components are needed for the realization of a two-stage heat exchanger—and, in other respects, this necessitates relatively improved separation of the high-temperature part and the low-temperature part. Depending on the type of an employed second and third fluid in the form of a coolant, the separation efficiency between the high-temperature part and the low-temperature part of the two-stage heat exchanger should be adjustable. Therefore, separation between an oil-based and water-based coolant, for example, should be especially good, whereas if the second and third fluid is formed in the form of similar coolants, leaks are basically tolerable, whereby however leakage rates between a high-temperature part and a low-temperature part are to be kept as low as possible.
Thus, for example, U.S. Pat. No. 5,755,280 discloses a heat exchanger, according to which the internal walls divide the interior of a housing and are themselves sealed from each other by means of round O-rings. On the other hand, German Pat. Application No. DE 103 28 746 A1 of the applicant, which corresponds to U.S. Publication number 20070125527, also discloses a concept in which mixing of cooling fluids is possible in each case and therefore separation of a high-temperature part and a low-temperature part can be omitted.